Automatic defrost control



Jan. 6, 1959 R. P. 4PERRY 2,867,092

AUTOMATIC DEFRosT CONTROL Filed Aug. 2e, 1954 2 sheets-sheet 1 IN VEN TOR.'

Jan. 6, 1959 R, P, PERRY 2,867,092

AUTOMATIC DEF'ROST CONTROL Filed Apg. 26, 1954 2 sheets-sheet 2 CUMPRESS? IN V EN TOR.'

AZZQRNEYS.

United States Patent AUTOMATIC DEFROST CONTROL Robert P. Perry, Greenville, Mich., assigner, by mesne assignments, to Hupp Corporation, Detroit, Mich., a corporation of Virginia Application August 26, 1954, Serial No. 452,267

'8 Claims. (Cl. 62-140) This invention is concerned with a refrigerator defrosting. system, and it relates particularly todefrosting systems in which a defrosting cycle is initiated automatically upon the accumulation of a frost coating of predetermined thickness upon the walls of the evaporator.

Anobject of this invention is to provide a defrosting system in a refrigerator wherein actuation of the system is initiated when a predetermined thickness of frost has accumulated upon and around thek environment of the refrigerant-bearing walls or conduits, and in which termination of the system is provided when the undesirable thickness of frost accumulation has been disposed of completely. Another object is to provide in such a system defrost control means capable of functioning to terminate the defrosting cycle upon disposal of the frost, and to return the refrigerant system to its normal refrigerating function.

Still another object is that of providing an arrangement of parts in which a member pivoting on a vertical axis secured to the defrost control element is operative to actuate the defrost control element only when the frost upon the evaporator has reached a predetermined thickness. Yet another object is to provide an arrangement of parts whereby intermittent soundings of the frost accumulation upon the evaporator are made for determining when the frost accumulation has reached a predetermined thickness, a defrost system being energized when such thickness is present.

A further object of the invention is to provide a member operatively arranged with mechanism in combination with a defrost control so that a movable pivoting element has its pivoting point transferred from the defrost control centrally of the pivoting element to the frost surface adjacent one end of the element when the frost has reached the predetermined thickness, spring torsion being developed through that transfer point to trip' the defrost control by reason of the stress placed thereon at the centrally located pivoting point. 4It is also a further object of the invention to provide an arrangement as above described whereby only marginal waste space within the refrigerator is necessary to house the defrost system, and to combine that system with conventional controls and solenoid valves commercially produced, and without sacrificing space or creating additional cost; and providing convenient accessibility for servicing, and to use the conventional arrangements of the present-day refrigerator to actuate the defrosting cycle when necessary, means also being provided for adjustment of the various parts and mechanisms to compensate for manufacturing variances that would adversely alfect critical adjustments and renderthe operation4 of the system ineffective.

Yet a further object is in providing a defrosting System'for refrigerators in which hot expanded refrigerant isy recirculated through the evaporator upon the initiation of a defrosting cycle to heatV the evaporator and thereby defrost the same. An-l additional object is in 2,867,092 Patented. Jan. 6, 19595 a valve controlled by-pass to the evaporator whenthe,4

defrosting system is actuated to quickly defrost the'y evaporator; and the refrigerator being returned tothe` operational conditions after the frost has been removedy l from the evaporator. Additional objects and.' advantages,

will appear as the specification proceeds.

An embodiment of the invention is illustrated in the accompanying drawings, in which-4 Figure l is a broken longitudinal sectional view a refrigerator embodying the invention; Figure l2 is ay vertical sectional view taken. on the vline 2 2 of Figure;

l; Figure 3 is a sectional View largely diagramatic of?,

the defrost control switch; Figure 4 is a schematic diaegram showing the wiring arrangement;r FigureV 5 is a-i broken longitudinal sectional View showing one position of the defrost control arm; Figure 6 is a View similar,

to that of Figure 5 and showing the sounding position of the arm; and Figure 7 is a further lview similar to those of Figures 5 and 6 and showing the control armi. in position to actuate the defrosting system, the accumulation of frost upon the evaporator Wall being of, sufcient thickness to bring about this result.

in describing the structure, reference Will first be made in particular to Figures 1 and2. these figures is an evaporator 10 having a wall 117v equipped with a corrugated outer wall portion 12 'de-t y ning a plurality ofy ow passages 13 extending aboutthe evaporator. As is weil known in the art, refrigerant,

is expanded through the ilowpassages 13,to extract heatv from the evaporator and to thereby chill the saine.,

The evaporator has an open forward end A14 and av compartment 15 is defined Within the interior of .the

closure door 16 is provided and is-hinged along oneM sideso that it can be swung to positions opening and@ The evaporator may be substantially vconventional in` all! respects, and such structures are. well` known in the art., 'y

The evaporator 1d is housed Within the'interior refrigf' erated compartment 17 provided by the walls 18 of a The walls k18 may be formed by spacingH apart an outer wall panel 19 and an inner wall panel f or liner 20, and by insulating the intermediate spacethere- Refrigerators hav!k 'y ing walls of this character are well knownn in the art,l and'- it is then believed unnecessary to' describe the refrigerclosing the access opening 14 of the evaporator.

refrigerator.

between with a thermal insulation 21.

ator and the walls thereof in further detail.

The liner 2U ata point opposite the wall 11 of the evaporator is provided with an opening 22 therethrough. j

and a bracket 23 is secured by cap screws 24! and through,

a bifurcated support 29 that provides a pivotal axis .30.

for a control arm 31 having a threaded laterallyl turned end portion 32 that threadedly receives thereon a pad 33. The end portion 32 isv turned toward the wall 110i" the evaporator, while the other end portion ofthe arm 31 is provided with a cam follower 34. or curved' endl portion which is turned in the opposite direction. YThe carri v Illustrated in follower' 34l is adapted to ride upon the cam or wedge 35 that is rigidly secured to the evaporator door 16 which is hinged along the side opposite that of the cam 35. The pivotal axis through the arm 31, and about which it is swingable, is vertically oriented and is substantially parallel to the wall 11 of the evaporator. The arm is adapted to pivot about that axis to swing the pad 33 against the wall 11 or against a frost accumulation thereonfand into the general position indicated by the dotted lines in Figure 1. When the evaporator door 16 is swung open, the cam 35 is Withdrawn from engagement with the cam follower 34 and the arm 31 pivots in a counter-clockwise direction, as viewed in Figure 1, about the axis'30 to move the cam follower into substantial abutl ment with the liner. 20. That movement is brought about by the biasing action of the leaf spring 36 which isnsecured at one end to the liner 29 by the cap screw 24, and which at its other end is.slotted and slidably receives within the slotthereof the cam follower 34 of the arm 31'.

"Theplunger 27 is shiftable toward and away from the wall 11 of the evaporator. When shifted away from theevaporator through a distance that is predetermined, theplunger is adapted to actuate a switch which when closed will initiate a defrost cycle in a manner to be described hereinafter. The switch components are illustrated best in Figure 3, and reference will now be made to that ligure in describing the control unit.

The plunger 27 is elongated and in its inner end 37 is axially slidable through an opening provided within abracket 38 that is rigidly secured to the casing of the control unit 26. Intermediate the ends of the plunger 27, it is provided with a laterally extending ear 39. That ear is adapted to engage the arm 40 of a switch lever 41 which is pivoted about the point 42. In the position ofthe switch shown in Figure 3, the plunger 27 is shifted toward the evaporator wall 11 and the switch lever 41 is inthe position in which the'switch contact 43, which is a stationary contact, and the switch contact 44, which is a movable contact, are separated. The contacts 43 and 44 are connected respectively to the terminals 45 and 46, which in turn are electrically connected to the leads 47 and 48 which comprise the electric conduit 49 (Fig. l).

The movable contact 44 is pivoted at the point 50 and carries an arm 51 which is pivotally mounted thereon and which in .turn is yieldably connected to the switch arm 41 by the spring 52. The spring 52, when the switch Y arm 41 is in the position shown, is operative to pull the upper end of the member S1 downwardly and thereby hold the contact 44 in spaced relation with the contact 43.' On the other hand, when the plunger 27 is shifted away from the wall 11 ofthe evaporator, the switch arm 41 is pivoted about the point 42 to swing the leg 40 thereof downwardly, whereby the tension of the spring 52 is relieved slightly and the member 51 becomes operative to swingnthe contact 44 into closing relation with the contact 43. Normally, the plate lever 53, which is pivoted about the-axis 54, is rotated slightly in a counterclockwise direction from the position shown in Figure 3 by the force of the bellows 55 which is equipped with an annular button 56 that engages a leg 57 of the plate member 53. When the plate lever 53 is in such position, the bellows 55 will be closed and will provide sucient force to maintain such position of the member 53 against the force of the coil spring 58, which at one end is secured to the lug 59 carried by the casing of the control lnit 26, and whichat its other end is secured to the plate ever.

The switch as described and illustrated is operative to maintain the contacts 43 and 44 in closed position once the switch lever 41 has been pivoted in a clockwise direction by the inward movement of the plunger 27 so long as the plate lever 53 is maintained in its most counter-clockwise position by the closed bellows 55 (the bellows 55 may be a Sylphon bellows).

When the bellows 5S is expanded as shown in Figure 3, the spring 58 is operative to pivot the lever 53 into the position shown in which it functions to separate the contacts 43 and 44.

The position of the bellows 55 is determined by the temperature of an expansion uid within the capillary tube 60, which at one end is in open communication with g the bellows and which is in heat exchange relation at its other end with the wall l1 of the evaporator, as is shown best in Figures Y1 and 2; That end of the capillary tube may be held against the evaporator wall 11 by the wing plate 61 which is anchored to the wall 11 by the cap screws 62. Thus, it is apparent that the condition of the bellows 55 will be determined by the temperature of the evaporator wall 11.

The wiring diagram for the refrigerator is illustrated in Figure 4 and in general the electrical system is substantially conventional. The refrigerator will be `provided with a compressor 63, which is motor-driven and is connected by the leads'64, 65 and 66 to a relay and terminal unit 67. A service plug 68 adapted to be received Within an electric outlet is connected by the leads 69 and 70 to the panel 67. A light 71 may be provided within the refrigerated compartment, and the usual push button switch 72 is in circuit with the light and is operated by the opening and closing of the refrigerator door. An adjustable control unit 73 is connected to the terminal panel 67 by the leads 74 and 75. The control member, as is well known, can be manually adjusted to determine the temperatures within the refrigerator compartment at which the compressor will be energized and de-energized. The circuit thus far described is conventional in all respects.

The control unit or defrost control Z6 through the leads 47 and 48 thereof is connected to the terminal panel 67. Connected to the terminal 46 of the control unit 26 is an electric conductor 76 which is connected to the lead 69 of the service plug, and has in series therewith a heating element 77 which may be a small resistance in the order of about 3 watts. The function of the heater 77 will be described subsequently. It is located physically, or is seen best in Figs. l and 2, about the capillary tube 60.

The circuit also includes a solenoid 78 which through leads 79 and 80 is connected to the terminal panelv 67. The solenoid is operative to control the position of a valve in a by-pass conduit 81. Normally, hot refrigerant which has been expanded through an evaporator (not shown) flows through the return conduit 82 from the evaporator and into the compressor 63. It is then pumped by the compressor through the conduit 83 which carries it to a condenser where it is chilled and, as is well known, subsequently recycled through the evaporator in another expansion step. Usually, the valve 84 is in the position shown in Figure 4 where it closes the conduit 81 and hot refrigerant is forced to ow through the passage 83. On the other hand, when the solenoid 78 is energized the valve S4 is moved to open the conduit 8l which is a by-pass around the condenser, and the hot expanded refrigerant liows through the by-pass conduit 81 from where it is recycled directly into the evaporator and heats the same.

Ordinarily, the refrigerator circuit permits the customary refrigeration cycle to be carried out. At such time the control unit 26 which, as has been described before, contains the contacts 43 and 44 therein is deenergized and those contacts are open. The solenoid 78 is de-energized and no current ows through the heater element 77. However, when the plunger 27 is shifted inwardly to close the-contacts 43 and 44, the compressor 63 is energized, the solenoid 78 is energized and current ows through the heating element 77. This Following through the circuit in terms of the arrangements of the elements therein, it is seen that the solenoid 78, switch unit 26 and thermostat control unit 73` are connected in a series arrangement. More specifically, starting with lead 69 from plug 68, it is apparent that lead 69 connects with lead 80 running to the solenoid 78; the circuit continues through the solenoid through line 79 leading therefrom to lead line 48 through the switch 26 and line 47 to the line 75 and thermostat 73. Lead line 74 extends from the thermostat 73 to line 70 and to the opposite side of plug 68.

Therefore, solenoid 73 is energized to open the bypass only when the switch 26 is actuated and only when the contacts of the thermostat 73 are closed. It will be apparent that the heater unit 7'7 is connected to opposite sides of the plug 68 in a series arrangement with the switch 26 and thermostat 73. Therefore, the energization thereof is controlled by the switch 26, just as the switch 26 controls actuation of the solenoid.

The "compressor 63 is connected to the plug 68 through the thermostat 73 and in a parallel relation with the defrosting circuits just described. Tracing the circuit through, it is found that lead 70-which at one end thereof is connected to the plug 68 is connected to one side of the thermostat 73 through the lead 74. Lead 7S connected to the opposite side of the thermostat is in turn connected to lead line 66 running to the compressor 63. Lines 64 and 65 running from the compressor are connected to terminals of the relay panel 67. Line 65 is the start line for the compressor and is connected to the terminal post for the lead 69 through the starting relay contained within the relay panel 67. The run line 64 from the compressor is connected to the terminal post of lead 69 through a connector bar within the relay panel 67. As a result of this circuit arrangement, the compressor is energized whenever the lcontacts of the thermostat switch 73 are closed.

The three primary positions of the arm 31 are shown in Figures through 7. The ordinary position is illustrated in Figure 5, and in the position there shown the cam 35 is in engagement with the cam follower 34, and the arm 31 is thereby swung in a clockwise direction to space the pad 33 from the wall 11 of the evaporator. When the evaporator door 16 is swung open the cam 35 is removed from engagement with the cam follower 34 and the arm 31 pivots about the axis 3i) under the influence of, the leaf spring 36 to rotate the pad 33 against the wall 11 of the evaporator. It has been brought out that the plunger 27 is shiftable, and then the influence of the spring 36 is operative to shift the plunger 27 away from the evaporator wall 1l. Since this shifting movement, if of sufficient length, will close the contacts 43 and 44, it is desirable to provide a stop which will limit such shifting and this stop is found in the liner which provides a stop or abutment for the cam follower 34. Thus normally, when there is no frost accumulation on the evaporator wall, or when the frost accumulation has not reached a predetermined thickness, opening of the evaporator door 16 will cause the arm 31 to assume the position shown in Figure 6. In that position the control unit 26 is not actuated and, in effect, the arm 31 has simply pivoted about the axis StB-the axis 30 and the plunger 27 which provides the same remaining relatively stationary.

if the evaporator door 16 is swung open when there is a frost accumulation S5 of predetermined thickness upon the wall l1 of the evaporator, the pad 33 will abut the same and wiii establish a new pivotal axis for the arm 3i. That new axis will oe the Contact surface between the frost accumulation SS and the pad 33, and the biasing force of the spring 36 will then be effective to shift the plunger 27 inwardly to a point where the leg i0 of the switch is depressed, and the contacts 43 and 44 within the control unit will then be closed. The re- 6 frigerator circuit as a result will beconditioned for' aI defrosting operation.

In operation of the system, the control unit 261 is nor-J mally in a tie-energized state and the cam 3S' 'willi holdk the pad of the control arm 3l. in spaced relation with the wall 11 of the evaporator. Upon each opening off the evaporator door 16, the arm 33 will be pivoted into the position shown in Figures 6 and 7 to take a'sounding of, or to measure the thickness of, the frost accumulationv upon the evaporator door- If no frost accumulation is present or if the frost accumulation has not attained a predetermined thickness, the pivotal axis 33 of the arm 31 `will govern its pivotal movement and the control unit 26 will remain de-actuated, and when the door 16 is again closed the arm 3l will be returned to the'position shown in Figures l and 5. This procedure is re-` peated each time the evaporator door is swung open.

When the frost accumulation 85, shown in FigureV 7, has attained a predetermined thickness, opening of the evaporator door 16 will cause the pad 33 to swing into engagement with that frost accumulation and the armv 31 will then be pivoted slightly about the axis dened between the frost accumulation and the pad 33 to vmove the plunger 27 away from the evaporator walland to then actuate the control unit 26. It may at this point be mentioned that tlie pad 33 is' adjustably positioned upon the arm 3l and, in the speciiic illustration given, theV pad is threadcdly mounted thereon. This adjustment permits ready change in the position of the pad upon the arm, and the control unit 26 may then be actuated by simply adjusting the pad to any desired frost thickness.

When the control unit 26 is actuated, the compressor 63 is started and the solenoid 7S is energized. At the same time, current flows through the heating element 77. lt is believed that the circuitry accomplishing these results is clear from the schematic showing of Figure 4', and that a detailed analysis of the circuit connections need not be presented.

Energizing the solenoid 78 opens the by-pass S1 andV hot or expanded refrigerant is then pumped directly intoV the evaporator and, of course, the walls thereof are heated. It will be appreciated that heating the evaporator walls will defrost the same.

Gnce the contacts 43 and 44 of the switch are closed, they will remain closed even though the evaporator door' 16 is returned to closing position and the control arm 31 swung to the position illustrated in Figures l and'S. The contacts will remain closed so long as the bellows is collapsed. The bellows will remain closed until the expansion iiuid in the capillary tube 60 attains some predetermined temperature, and 'when vthat temperature is attained, the bellows will expand to free the plateA lever 53 which then will pivot under the inuence ofi the spring 53 into the position shown in Figure 3, and the contacts 43 and 44 will be opened and the defrosting cycle then terminated. v

The capillary tube 6d is in heat exchange relation with the wall 11 of the evaporator and as that wall is heatedv by the recycling therethrough of the expanded refrigerant, the expansion fluid within the capillary tube will be heated and the bellows will begin to expand. However, the temperature of any portion of the expansion fluidi 'within the capillary tube will govern the extent of expansion of the bellows 5S, and even though the evaporator wall 11 may be heated to a temperature suliicient to cause defrosting and suliicient to normally expand the, bellows, a cold point along the capillary tubeV would prevent the expansion of the bellows. This then would defeat the intention of separating the contacts'43 and 44 to terminate the defrosting cycle. That cycle then might continue operating for a longer periodtthan necessary,y to defrost the refrigerator. This result is avoided,` however, by simply employing the resistance heating element 77 for the current flow therethrough will provide* the necessary cheat forexpanding the lluid .withinthe capillal'yr anemona.`

tube even though there would normally vbe a cold point therealong which would cause an undesirable lag in the expansion of the bellows. Thus, it will be seen that I have provided a system for defrosting a refrigerator, the positive initiation of the defrosting cycle being dependent upon the presence of a frost accumulation in that thickness at which the system has been selectively set to initiate the defrosting cycle, and to thereafter terminate such defrosting cycle upon a predetermined ternperature rise in the evaporator walls over a period of time known to be sufficient to have disposed of or to have melted the frost accumulation thereon.

While in the foregoing specification an embodiment of the invention has been set out in considerable detail for purposes of illustration, it will be apparent to those skilled in the art that numerous changes may be made in those details without departing from the spirit and principles of the invention.

I claim:

l. In combination with a refrigerator having a doorequipped evaporator, an elongated arm equipped adja cent one end with a pad and adjacent its other end with a cam follower, a support intermediate the ends of said arm and being shiftable toward and away from said evaporator, said arm being pivotally carried by said support about an axis enabling said pad to be swung toward and away from said evaporator, means for biasing said arm to bring said pad into engagement with said evaporator and for biasing said support away from the evaporator, cam means carried by said door and being engageable with said cam follower, when said door is closed, to pivot said arm about said axis to move said pad into spaced relation with said evaporator, and control means operatively arranged with said support in response thereto upon the shifting thereof a predetermined distance from said evaporator.

2. In combination with a refrigerator having a doorequipped evaporator, an elongated control arm equipped at one end with means for measuring the thickness of a frost accumulation upon said evaporator, said control arm being pivotally mounted intermediate the ends thereof, cam means carried by said door and being engageable, when said door is closed, with said control arm to pivot the same in a direction to space the frost measuring means from said evaporator, and defrosting means operatively arranged with said control arm and being actuated thereby when the frost accumulation upon said evaporator reaches a predetermined thickness.

3. In a refrigerator having an evaporator equipped with a swingable door, a control unit mounted within said refrigerator and providing a support member, an elongated control arm pivotally mounted intermediate the ends thereof upon said support member, said control arm being equipped at one end with a pad engageable with said evaporator or with a frost accumulation thereon, said control arm being provided at its other end with a cam follower, cam means carried by said door` and being engageable with said cam follower to pivot said control arm in a direction to space said pad from said evaporator, biasing means, arranged to be overcome by said cam means, for normally biasing said control arm to move said pad toward said evaporator, said support being shiftable toward and away from said evaporator, and switch means provided by said control unit for actuation by the shifting of said support member.

p f 4'. In an arrangement for defrosting a refrigerator equipped with an evaporator provided with a closure door and having a refrigerant flow system in which refrigerant is expanded through the evaporator and then recirculated after the cooling and compressing thereof, a by-pass in said system for recirculating expanded refrigerant through said evaporator, control response means for selectively opening said by-pass, and control means for operating said control response means at selected times to open said by-pass and recirculate ex- CTL panded refrigerant through said evaporator to defrost the same comprising an elongated arm equipped adjacenti one end thereof with a pad and adjacent its other end with a cam follower, a support intermediate the ends of said arm and being shiftable toward and away from the evaporator of said refrigerator, said arm being pivot ally carried by said support about an axis enabling said pad to be -swung toward and away from the evaporator, means for biasing said arm to bring said pad into engagement with the evaporator and for biasing said support in a direction away from the evaporator, and means carried by the evaporator door and being engageable with said cam follower, when said door is closed, to pivot said arm about said axis to move the pad into spaced relation with the evaporator, and means operatively arranged with said support and being responsive thereto upon the shifting of the support a predetermined distance from the.

evaporator for energizing said control response means.

5. In combination with a refrigerator having a doorequipped evaporator through which a refrigerant is expanded to cool the same, a defrosting system for said evaporator comprising a by-pass in the refrigerant flow circuit of the refrigerator for recirculating expanded refrigerant through said evaporator, valve means for controlling said by-pass, and control means for opening said valve at predetermined times to recirculate expanded refrigerant through said by-pass and into said evaporator to elevate the temperature thereof comprising an elongated arm provided at one end with a cam follower and at its other end with a pad adapted to engage the evaporator, shiftable support means movable selectively toward and away from saidevaporator, said arm being pivotally mounted intermediate the ends thereof on said support means, the door of said evaporator being equipped with means engageable with said cam follower for pivoting said pad away from the evaporator when the door is closed, and switch means operatively arranged with said support means and being actuated upon a shift thereof to a predetermined position.

6. in a refrigerator having an evaporator, a refrigerant flow system therethrough and means for restoring refrigerant to a pre-expanded condition following the expansion thereof through said evaporator, conduit means for recycling refrigerant expanded through said evaporator back thereto prior to its restoration to pre-expanded condition, a solenoid control valve for said conduit means for controlling the tiow of refrigerant therethrough, means for measuring the thickness of a frost accumulation upon said evaporator, switch means operatively arranged for actuation thereby when a frost accumulation upon said evaporator reaches a predetermined thickness, and circuit means including said switch means and said solenoid for energizing the latter when said switch means are actuated, said means for'measuring the thickness of a frost accumulation comprising a control arm having at one end means for measuring the thickness of a frost accumulation upon said evaporator, said control arm being mounted for pivotal movement intermediate the ends thereof whereby said last mentioned means are movable into engagement with a frost accumulation upon said evaporator, said switch means providing the pivotal mounting for said control arm and being shiftable into a plane substantially normal to that axis, the pivotal axis of said control arm being shiftable toward the end thereof equipped with the means for measuring the thickness of the frost accumulation to shift said switch means and thereby actnate the same.

7. In combination with a refrigerator having an evaporator, control means for initiating a defrosting cycle for said refrigerator and including means for terminating a defrosting cycle when the temperature of the evaporator approaches a predetermined value, said latter means comprising a heat responsive device equipped with a capillary tube lled with an expansion fluid, said tube being carried in heat exchange relation with said evaporator whereby the evaporator temperature establishes the condition of said heat responsive device, and switch members controlled by said heat responsive device for terminating a defrosting cycle, and a relatively small heating element mounted in heat exchange relation with said capillary tube, said element being energized during the defrosting cycle to heat the tube and eliminate cold points therein so that the condition of said heat responsive device is determined by the temperature of said evaporator.

8. A combination of the character set forth in claim 7 wherein the said heating element is in heat exchange relation with the portion of said capillary tube between said evaporator and said switch members.

References Cited in the flle of this patent UNITED STATES PATENTS Hoffman Nov. 25, 1930 Keighley Nov. 26, 1935 Volpin Feb. 2l, 1939 Kurtz Nov. 21, 1950 Janos et al. Nov. 25, 1952 Grimshaw Jan. 6, 1953 Smith Mar. 24, 1953 Jones Jan. 19, 1954 

